Can a job force you to cover up a small tattoo?

Generally, yes. The simple answer is for you to likely just cover it. You likely won't want to risk a termination, and further this isn't likely to be worth pursuing via a lawsuit. If the employer fired you for having a displayed tattoo that they had know about and accepted for many years, then you may have a wrongful termination claim, but even that wouldn't be a particularly good one. If you refuse to comply with a request from your supervisor, though, that alone can be adequate reason enough for termination. Likely your employer has a written dress code policy, which you should review. Even that can change over the years, however, so being in compliance 10 years ago does not necessarily mean you're in compliance now. There may be other facts that could impact your decision as to how to respond, of course, but if you can proactively avoid an issue, its usually best to do so. Of course, if you have other more specific concerns, or if you otherwise plan to refuse to comply with the request, you should consult with and retain an attorney to represent you as soon as possible.